Chemical looping combustion (CLC) and chemical looping gasification (CLG) represent promising technologies for capturing carbon dioxide (CO2) emissions from fossil fuels with reduced energy penalty. These processes utilize the transfer of oxygen from air and/or water to the fuel through cyclic redox operations of solid oxygen carriers. Similar cyclic redox processes include the steam-iron process, solar-thermal water-splitting processes, and a number of redox catalyst-based selective oxidation processes, e.g., the DuPont redox maleic anhydride production process.
An important aspect of these cyclic redox processes is the selection of oxygen carrier (or redox catalyst) materials with adequate reactivity. Oxides of transition metals (e.g., Mn, Fe, Co, Ni, V, Mo, and Cu), their mixtures, and natural minerals have been used as oxygen carriers in CLC. High reactivity during redox reactions over a long term and the ability to fully or partially convert the fuel are sought-after characteristics. In addition, thermal stability, mechanical strength, and resistance to agglomeration also are important. Two methods for achieving the desired characteristics of stability, strength, reactivity, and selectivity are known. One method is to mix the oxygen carrier with an inert support, such as TiO2, SiO2, ZrO2, Al2O3 or MgAl2O4, followed with thermal treatment.
To date, however, an effective strategy to design oxygen carrier/redox catalysts with optimum performance in terms of stability, strength, reactivity, and selectivity is still lacking.